1. Field of the Invention
The present invention relates to a copper-based sintered contact component which has exceptional resistance to wear and fusion and can stably retain a comparatively high coefficient of friction, say, about 0.4 to 0.6, under dry contact conditions. The component can stably exhibit a friction coefficient of more than 0.1 when placed in lubricating oil. Further, the component is very unlikely to attack counterpart material and has high strength, high toughness, and high hardness. Therefore, it is applicable for use in the art of contact components, such as clutches and brakes of both dry and wet types.
2. Description of the Prior Art
Recently, for use as materials for friction clutches and brakes to be used under dry conditions, bronze-based sintered alloys have been developed which can replace asbestos-based friction materials. For example, Japanese Patent Application Laid-Open No. 58 -126948 describes a "dry sintered friction material" which comprises a bronze-based sintered alloy and hard particulate matter added thereto to provide higher coefficient of friction.
However, such sintered material has no reaction layer between the hard particulate matter dispersed in the alloy and the matrix, but involves gaps therebetween. As such, during friction contact under high-speed, high-load contact conditions, hard particles may come off the matrix, and this makes it impracticable to expect any high friction coefficient to be stably exhibited. Another problem is that fusion may occur relative to counterpart material starting from the point at which such particle fall-off has occurred. A further problem is come-off particle bite and attack against counterpart material.
Additionally, in the aspect of mechanical characteristics, the sintered material as an abrasion material involves the problem that aforesaid gaps will deteriorate the mechanical characteristics of the material, such as strength, toughness and hardness. Another problem is that the dispersed hard particles are diametrically large-sized on the order of 30 to 80 .mu.m, which fact, as a source of fracture, will induce deterioration in strength and toughness.
Conventional wet friction materials which have generally been used are porous paper-made friction materials and/or carbon sintered materials. As an example of the first mentioned type, reference may be made to Japanese Patent Application Laid-Open No. 6-25653 which describes a "paper friction material" composed principally of a thermosetting resin, such as phenolic resin, and friction adjustors, such as graphite powder and organic dust, with organic fibers and carbon fibers used as reinforcements. For the latter mentioned type, reference may be made to Japanese Patent Application Laid-Open No. 4-76086 entitled "Wet Friction Material" in which is proposed a carbon fiber-reinforced carbon sintered body produced by sintering a composite composed of uncarbonized carbonaceous fibers and carbonaceous powder. Materials of the both types are elastically deformable, and this enables any biased engagement force to be absorbed on the friction material side.
Generally, however, such friction material, when held in oil, has a small coefficient of friction only on the order of 0.1 to 0.15. Therefore, in order that a clutch, for example, may provide any sufficient transmission torque, it is necessary that a friction material having larger diameter and larger surface area be used. As a result, the current need for size/weight reduction cannot be met. Further, the paper friction material lacks heat resistance. During friction contact under high temperature conditions, therefore, such friction material may become damaged due to frictional wear and may also be subject to deterioration in its characteristics, which will lower the friction coefficient of the friction material further.
In an attempt to overcome these problems, Japanese Patent Application Laid-Open No. 4-76086, entitled "Friction Material", proposes a friction material such that a pseudoalloy spray coating comprised of two kinds of metal is formed on the contact surface of a friction material (glass fiber and/or rubber binder), whereby the friction material can have a high friction coefficient of the order of more than 0.25.
According to this technique, however, the process of spray coating is required in addition to the placement of the friction material on a substrate. This poses a problem because it adds to costs and process complexity. Further, because of the fact that thermal spraying is required, it is impracticable to expect any good productivity with the method.
Conventionally, bronze-based (copper-tin) sintered alloys have been used as oilless bearing materials. Sintered alloys of this type have high abrasion resistance and high fusion resistance. Therefore, it may be expected that a friction material comprised of such a sintered alloy with hard particles loaded and dispersed therein would exhibit a high coefficient of friction without being liable to any damage due to friction.
As already stated, however, it has been found that since hard particles loaded are present in grain boundaries of the matrix bronze powder, during friction contact, such particles would fall off and even attack the counterpart material, and that the sintered material per se would be subject to fusion and frictional wear. Another problem is that since bearing-type sintered alloys have many vacancies dispersed in their interior, in a high slide velocity range in excess of 20 m/sec, for example, lubricating oil present within the sintered alloy flows out of the interior so that an oil reservoir may be formed in a vacancy in the contact surface. As a consequence, a thick oil film is formed between the contact surface of the friction material and the counterpart material, with the result that the friction coefficient of the friction material is lowered to a 0.01-0.02 level.